Several years ago, there were horror stories in various legal periodicals about confidential material that was disclosed to the opposing side in sensitive negotiations. It seems that counsel for one side had sent drafts of documents via email in Word to his client. The client made changes and comments in the Word file and emailed them back to the attorney. The attorney incorporated the changes and emailed them to the opposition.
The opposition, by use of the "track changes" feature in Word was able to see the history of the document beginning with the original draft, moving through the opposing party's changes and comments, and concluding with the draft actually submitted. This gave the opposition some fairly good insight into the other side's negotiating posture and strategy.
Apparently, some people never learn from the mistakes of others. It appears that the Mehlis Report, produced as a result of the investigation as to who was behind the assassination of former Lebanese Prime Minister Rafik Hariri, did not intend to set forth the names of those behind the killings. Instead, the final report was intended to only refer to "senior Lebanese and Syrian officials" as those who decided to assassinate Hariri. However, the change was made in the same Word document which, in an earlier iteration, had set forth the names of the officials involved. By using the "track changes" feature, the names were publicly revealed. The complete story is set out in Mark A. LeVine's (no relation) posting on his blog at the History News Network.
This is but one reason to (i) use WordPerfect rather than Word or (ii), at the least, send out only documents that have been converted to pdf.